Coroutines

Write async processing with coroutines and Flow.

Suspend calls with coroutines

The fourth lesson makes UserRepo and Api methods suspend. This mimics Ktor's suspend-based API. main now opens with runBlocking.

Example: suspend fun save(u: User). suspend fun find(id: Int): User?. main = runBlocking { repo.save(...); api.route(...) }. suspend functions are called inside a coroutine. This pattern is suitable for concurrent I/O; it does not block threads.

runBlocking and coroutine scope

runBlocking opens a coroutine scope and blocks. suspend functions are called like normal functions inside this scope.

Example: fun main() = runBlocking { repo.save(User(1, "Alex")); println(api.route("/users/1")) }. runBlocking keeps main from finishing until the coroutine completes. This pattern is the standard way to test concurrent code synchronously.

Exercise Convert methods to suspend

Keep all existing definitions. Add suspend before UserRepo.save, UserRepo.find and Api.route. Turn main into fun main() = runBlocking { }; move repo.save(User(1, "Alex")) and println(api.route("/users/1")) into it. Add import kotlinx.coroutines.runBlocking.

Show example solution

import kotlinx.coroutines.runBlocking

data class User(val id: Int, val name: String)

object UsersTable {
    val rows = mutableListOf<User>()
    fun insert(u: User) { rows.add(u) }
    fun select(id: Int): User? = rows.find { it.id == id }
}

class UserRepo {
    suspend fun save(u: User) { UsersTable.insert(u) }
    suspend fun find(id: Int): User? = UsersTable.select(id)
}

class Api(val repo: UserRepo) {
    suspend fun route(path: String): String {
        return when {
            path.startsWith("/users/") -> {
                val id = path.removePrefix("/users/").toInt()
                repo.find(id)?.name ?: "not found"
            }
            else -> "404"
        }
    }
}

fun main() = runBlocking {
    val repo = UserRepo()
    repo.save(User(1, "Alex"))
    val api = Api(repo)
    println(api.route("/users/1"))
}

Test yourself

How is a suspend function called in main?
- Inside runBlocking
- With Thread.start
- With a Timer
- Directly
Show answer

Inside runBlocking — A suspend function is called inside a coroutine like runBlocking.
Match the coroutines parts

suspend fun

Awaitable function
runBlocking

Scope opening
repo.save(...)

Call
main = runBlocking

Entry point

Complete the suspend function definition

class UserRepo {
    ____ fun save(u: User) { UsersTable.insert(u) }
}

Show answer

class UserRepo {
    suspend fun save(u: User) { UsersTable.insert(u) }
}

Next lesson: Putting it together

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